A conventional ring oscillator has a problem that phase noise is liable to become large due to an effect of a mixed signal from an outside because internal signals are not complementary to each other in a configuration that an odd number of inverting amplifiers are connected in a ring in a case where this oscillator does not employ differential amplifiers. Further, use of differential amplifiers decreases effects of the mixed signal from the outside but, at the same time, increases phase noise due to thermal noise, problematically.
In a conventional quadrature modulator, to obtain modulation signals orthogonal to each other, it has conventionally been performed to shift a phase of a modulation input by 90 degrees by using a phase shifter so that signals whose phases are shifted by 90 degrees from each other may be obtained or to divide a frequency of the modulation input so that the signals whose phases are shifted by 90 degrees from each other may be obtained.
In a method for shifting the phase by 90 degrees by using a phase shifter, in order to shift the phase by 90 degrees accurately, it is necessary to improve a precision of components of the phase shifter or adjust the phase shifter by trimming or perform automatic calibration on the phase shifter. However, an approach for improving the precision of the components of the phase shifter has a problem that it is difficult to implement in an IC and not cost effective. An approach for trimming the phase shifter has a problem that trimming involved is expensive. An approach for automatic calibration of the phase shifter has a problem of an increase in costs because it needs to have means for detecting a shift in phase by the phase shifter accurately and means for automatic calibration.
In a method for dividing a frequency of a modulation input to obtain modulation signals having phases shifted by 90 degrees from each other, a modulation input signal having a higher frequency than that of the modulation signals is required, so that in particular when the modulation frequency is high, it is necessary to use a device that can follow this high frequency and generate a high-frequency modulation input, which increases costs or power dissipation, problematically.
As for a voltage controlled oscillator, on the other hand, a method, among others, of using a ring oscillator needs to use an even number of stages of ring oscillators in order to directly provide signals having phases shifted by 90 degrees from each other, in which an even number of differential amplifiers have been connected in a ring. Since the differential amplifiers have been used, phase noise of the ring oscillator has become even larger.
The present invention implements two objects of (1) realizing a ring oscillator that has comparatively small phase noise and is not so affected by a mixed signal from an outside and (2) obtaining modulation signals having phases shifted by 90 degrees from each other accurately in order to realize an accurate quadrature modulator.